All axes and series have methods which provide various customization options for the labels:

setLabelColor(int): used to change the color of the label, has a counterpart — getLabelColor()

setLabelSize(float): used to change the size of the label, has a counterpart — getLabelSize()

setLabelMargin(float): used to change the margins of the label, has a counterpart — getLabelMargin()

setLabelFont(Typeface): used to change the font of the label, has a counterpart — getLabelFont()

setLabelFormat(string): used to change the format string of the label, has a counterpart — getLabelFormat()

Additionally, you can provide a custom label render in order to completely change the way the labels are rendered.
In order to set the renderer you need to use the setLabelRenderer(ChartLabelRenderer) method.
Here's an excerpt from a custom label renderer: